The present invention relates to an organic electroluminescence (EL) device and a method for driving same.
FIG. 6A shows a schematic diagram of a conventional organic electroluminescent (EL) device 600, which is operated under a constant current applied thereto. As shown in FIG. 6A, the organic EL device 600 includes a cathode 101, a light emitting layer 102, an organic hole carrying layer 103, a transparent anode 104 and a glass substrate 105. In the organic EL device 600, the light emitting layer 102 and the organic hole transfer layer 103 are formed between the cathode 101 and the transparent anode 104 as shown in FIG. 6A. The transparent anode 104 is disposed on the glass substrate 105 and the cathode 101 and the transparent anode 104 are connected to a power source 110.
The light emitting layer 102 may be made of an organic fluorescent film, e.g., the so-called Alq3 (tris(8-quinolinolato) aluminum); the organic hole transfer layer 103 may be made of a triphenylamine. The cathode 101 is a metallic electrode, which may be made of an alloy, e.g., Mgxe2x80x94Ag or Alxe2x80x94Li, and the transparent anode 104 may be made of an Indium Tin Oxide (ITO). There has been also known an organic EL device, wherein an organic electron transfer layer is formed between the cathode electrode 101 and the light emitting layer 102.
FIG. 6B reveals a partial cutaway view of the conventional organic EL device 600 for use in a dot-matrix type display. In this EL device, light emitting portions are defined by the cathodes 101 and the transparent anodes 104 facing each other and having the light emitting layer 102 and the organic hole transfer layer 103 therebetween. Each overlapping region of the cathodes 101 and the anodes 104 constitutes a pixel of the light emitting portions.
Such an organic EL device is a self-luminescent display device capable of being driven by a DC voltage. The organic EL device is of a thin and light flat panel display, having a large viewing angle, high brightness and a high impact resistance since the organic EL device is a solid-state device. The luminescence of the organic EL device is proportional to an integrated value of currents applied thereto. The organic EL device has high responsiveness and high luminescent efficiency. The organic EL device can achieve a luminescence level of, e.g., 1000 cd/m2 when a DC voltage of 10 V is applied between an anode and a cathode thereof for low voltage driving thereof.
Since, however, the organic EL device is formed with very thin films, there may easily occur micro-shorts due to a surface roughness of the transparent electrode or inclusion of impurities. If a short occurs at a single spot in the circuit of the organic EL device, the current is concentrated thereon, thereby greatly affecting the luminescence and being unable to turn on the light emitting portions along the line where the short occurred, which results in the yield of the device being deteriorated.
In a display device such as a vacuum fluorescent display device or a liquid crystal display device, a constant voltage driver is usually employed in lieu of a constant current driver, which is costly and less available.
However, the use of less costly constant voltage driver in the organic EL device entails certain problems that the luminescence level thereof varies a lot with the temperature change thereof. Referring to data and graphs corresponding to non-compensation items in Table 1 and FIG. 3 to be described later in detail, respectively, even when the temperature of the organic EL device is increased by only about 20xc2x0 C. (i.e., from 30xc2x0 C. to 50xc2x0 C.), the luminescence level thereof is increased about 2.1 times. Further, if a voltage applied to the organic EL device is increased, durability of the device is decreased.
The use of a constant voltage driver in an organic EL device, having luminescent elements disposed in a matrix form and employing a highly resistant transparent conductive ITO film as an anode wiring thereof, entails a luminescence gradient to occur between an upper part and a lower part of the matrix due to the voltage drop in the ITO film. Further, there occurs a great luminescence change within a operating temperature range due to intrinsic temperature dependency of the organic EL device.
For example, an organic EL device of an average luminescence level of 300 cd/m2 with a duty ratio 1/240 for a dot of 0.3 mm2 requires an instantaneous luminescence level of 72000 cd/m2. When an Alq3 is used as a light emitting layer, a current of 2.4 mA is required to flow through a dot of 0.3 mm2. When a sheet resistance of an anode ITO is 20 xcexa9 and a length of wiring between an upper most dot and a lower most dot thereof is 72(=0.3xc3x97240) mm, a wiring resistance becomes 5 kxcexa9. In this case, if a current of 2.4 mA flows, a voltage drop becomes 12 V and there occurs a luminescence difference greater than a factor of {fraction (1/10)} between the upper most dot and the lower most dot thereof.
It is, therefore, an object of the present invention to provide an organic electroluminescece (EL) device and a method for driving same, wherein the organic EL device is driven in a constant voltage mode by using a constant voltage driver. The organic EL device has a monitoring section outside a light emitting section, wherein a variation of an internal resistance in the monitoring section due to a temperature change is detected by using a current therethrough and is fed back to a driving voltage of a power supply.
In accordance with one aspect of the present invention, there is provided an organic electroluminescence (EL) device including:
a display section having one or more light emitting units; and
a monitoring section positioned outside the display section and having one or more monitoring cells,
wherein each of the light emitting units and the monitoring cells includes a cathode, an anode and at least one organic EL layer positioned between the cathode and the anode, either cathodes or anodes of the light emitting units and the monitoring cells being transparent, and wherein a current passing through an anode and a cathode of a monitoring cell is monitored to control the light emitting units.
In accordance with another aspect of the present invention, there is provided a method for driving an organic electroluminescence device having a monitoring section and a light emitting section, the method including the steps of:
flowing a constant current through the monitoring section;
monitoring a voltage due to the constant current; and
applying an operation voltage to the light emitting section, the operation voltage being obtained by a feed-back of the monitored voltage.